Java 8 parallel stream of list of method param

Answer 1

Looks very verbose, but you can try the following. The runAsync() method will make the list chunks run in parallel.

private void test(List<Object> list, int chunkSize) throws ExecutionException, InterruptedException {
    AtomicInteger prev = new AtomicInteger(0);
    List<CompletableFuture> futures = new ArrayList<>();
    IntStream.range(1, (int) (chunkSize * (Math.ceil(Math.abs(list.size() / (double) chunkSize)))))
            .filter(i -> i % chunkSize == 0 || i == list.size())
            .forEach(i -> {
                List<Object> chunk = list.subList(prev.get(), i);
                futures.add(CompletableFuture.runAsync(() -> invokeList(chunk)));
                prev.set(i);
            });
    CompletableFuture.allOf(futures.toArray(new CompletableFuture[0])).get();
}

private void invokeList(List<Object> list) {
    System.out.println("Invoked for: " + list);
}

I ran it for a list of 30 integers, with a chunk size of 5 like this:

public static void main(String[] args) throws ExecutionException, InterruptedException {
    List<Object> list = IntStream.range(0, 30).mapToObj(i1 -> (Object) String.valueOf(i1)).collect(Collectors.toList());
    int chunkSize = 5;
    new Test().test(list, chunkSize);
}

Output:

Invoked for: [15, 16, 17, 18, 19]
Invoked for: [0, 1, 2, 3, 4]
Invoked for: [5, 6, 7, 8, 9]
Invoked for: [10, 11, 12, 13, 14]
Invoked for: [20, 21, 22, 23, 24]

Answer 2

Guava has methods Lists.partition and Iterables.partition that do something like what you're asking. Say you have a large List and want to process it in chunks of 5, you could do:

int batchSize = 5;
Lists.partition(list, batchSize)
   .parallelStream()
   .forEach(batch -> invokeList(batch));

Answer 3

if you don't want to bring additional dependencies like Guava, then you can write a collector which divide your list in chunks:

static <T> Collector<T, List<List<T>>, List<List<T>>> toChunks(int size) {
    return Collector.of(ArrayList::new, (list, value) -> {
        List<T> chunk = list.isEmpty() ? null : list.get(list.size() - 1);
        if (chunk == null || chunk.size() == size) {
            chunk = new ArrayList<>(size);
            list.add(chunk);
        }
        chunk.add(value);
    }, (list1, list2) -> {
        throw new UnsupportedOperationException();
    });
}

and then call it as follows:

 List<Integer> list = Arrays.asList(1,26,17,18,19,20);
 list.stream().collect(toChunks(5))
              .parallelStream()
              .forEach(System.out::println);